This invention relates to a fixture for accurately positioning and supporting an integrated circuit chip testing board during the testing of an on-board integrated circuit chip by a design verification system.
It is desirable to test very complicated integrated circuit chips during the prototype stages of development and during mass production so that the integrity of their design and manufacture may be verified. To this end complex design verification systems have been developed which selectively excite the inputs of an integrated circuit chip with many input combinations and simultaneously check the outputs for accuracy. For each input combination, there is an expected output combination. If there is a discrepancy between the expected output and the actual output for a particular input combination, there may be a design error present in the chip. By testing many input combinations, the likelihood that a fault in the chip will pass undetected is significantly reduced, if not eliminated.
Such a design verification system includes a testing unit with sophisticated electronic circuitry and software for generating the chip excitation signals and comparing the actual chip outputs with the expected outputs as well as mechanisms for electrically connecting the testing unit to the pins of the integrated circuit chip. These mechanisms typically include a testing platform attached to and electrically connected with the testing unit and a testing board, also called a device-under-test (DVT) board, removable from the testing unit to which an integrated circuit chip may be attached. The testing platform has many electrical connectors protruding from its top which are typically arranged in a plurality of concentric circles. The side of the testing board opposite the side to which the chip is mounted has many metal pads or electrical contact points, which are also arranged on the board in concentric circles to correspond to the arrangement of electrical connectors so that when the testing board is placed onto the testing platform, each electrical connector on the platform will make contact with a respective contact point on the board. Each of the pins of the chip is electrically wired to a respective contact point on the opposite side of the board.
The electrical connectors used on the testing platform are often compressible pins. Such a pin has an internal assembly which consists of an internal casing containing a spring, a ball bearing, and a shaft connected to the head of the pin. When force is exerted on the pin head, the shaft slides further into the internal casing and compresses the spring. The distance through which the pin head may be compressed is limited by an annular stop on the inside of the internal casing.
There are typically many such pins attached to the testing platform. For example, one known design of a testing platform contains an array of up to 512 pins which are arranged in eight generally concentric circles with 64 pins in each circle.
The mechanisms for connecting the testing unit to the integrated circuit chip also include a means for clamping the testing board to the testing platform on the testing unit. A plurality of thumbscrews located about the periphery of the testing board are typically screwed through the board into a number of respective threaded bores in the testing platform to hold the testing board in place.
There are a number of problems associated with the use of thumbscrews to hold the testing board in place. First, it is desirable to depress each of the compressible pins by the same amount so that uniform electrical contact between the heads of the pins and the contact points on the testing board is achieved. However, when a plurality of thumbscrews are used to hold the board in place, there is no guarantee that each of the thumbscrews has been screwed into the testing platform to the same extent. If some screws are screwed into the testing platform more than others, the testing board will be skewed with respect to the testing platform, thus causing non-uniformities in the displacement of the compressible pin heads. This may result in non-uniformities in the electrical contact between the pins and the respective contact points, and may even result in some of the pins failing to make contact with their respective contact points, thus resulting in faulty indications of design errors when the chip is subsequently tested.
Another problem stemming from the use of thumbscrews to hold the testing board in place lies in the possibility that one of the metal thumbscrews may be inadvertently dropped onto the testing board after it is placed onto the compressible pins. If the testing unit power is on, the dropped thumbscrew might provide a conducting bridge between some of the metallized circuits on the testing board and cause a short circuit, possibly damaging the testing unit and/or the prototype integrated circuit chip.